Donatello, a mutant red-eared slider, holding a non-mutant red-eared slider.
The red-eared slider is a semi-aquatic species of terrapin indigenous to North America, specifically the southern United States and northern Mexico. They are so-named because they have a small red stripe by their ears. The red-eared slider is the most popular species of pet turtle in the United States.
In the original Eastman and Laird's Teenage Mutant Ninja Turtles comic book series by Mirage Studios, the four titular characters Leonardo, Raphael, Donatello and Michelangelo are all mutated red-eared sliders. The red bandanas they all wear are an allusion to the species.
Other versions of Teenage Mutant Ninja Turtles have been more ambiguous about the exact turtle species of their four main characters, or their stories suggested they were different species altogether. Indeed, most other versions of the four turtles wear color-coded bandanas, with no allusion to the red-eared slider.
In Rise of the Teenage Mutant Ninja Turtles, Leonardo is once again a red-eared slider, though his three brothers are each a completely different species of turtle. Leo wears a blue bandana, though his red markings are clearly visible in a way never before portrayed in a mutant turtle design.